I can't imagine having a child at 22, and I think that you and your partner are perfectly entitled to take your time and live your life together.
While I love my children, they do change your life dramatically. It's perfectly reasonable to decide against parenthood. Or to simply choose to wait 10 or 15 years.
So I'd simply say, "We're not ready." And smile, if you can.
If they push it? I'd pull the pushiest of the people aside and say, "I respect Jim and John and Joe's choices to have their family at a young age. But we have other plans. You're making me uncomfortable by continuing to ask about our family. We may have them in ten years. We may never have children. But it's our decision, and I'd like you to respect that."
Afterwards? In group settings, you can quietly say, "I think we've made ourselves clear." And then excuse yourself.
Or, hey, humor might work. My (dear, departed) grandmother used to say, "I want to see all of my great-grandchildren before I die." I used to smile and tell her to plan to live to be 110. (She was in her 70s when her last granddaughter - and first great-grandchild - was born, so really ... it wasn't entirely a joke!)
It's unfortunate that your partner's family isn't supportive. Dignity and directness will serve you well, if humor fails.
Good luck!